1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) display device and method for manufacturing the same. More particularly, the present invention relates to an organic EL display device having improved light coupling efficiency.
2. Discussion of the Background
Generally, an organic EL display device is a self-emissive display that emits light by electrically exciting a fluorescent or phosphorescent organic compound. It may be thin, driven by a low voltage, and have a wide viewing angle and fast response speed. Hence, the EL display may solve problems found in existing liquid crystal displays. Therefore, it has attracted attention as a next-generation display.
A stack-type organic EL display device has been developed and commercialized as a green light emission display having an improved life span. Novel organic materials having a wide variety of molecular structures have been developed, and research into self emitting color display devices continues.
Generally, an organic EL display device is manufactured by forming an organic layer, having a predetermined pattern, between electrode layers on a glass or transparent insulating substrate. Examples of materials that may be used for the organic layer include copper phthalocyanine (CuPc), N,N′-Di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB), tris-8-hydroxyquinoline aluminum, and other like materials.
The organic EL display device forms an image based on the following principle. As positive and negative voltages are applied to electrodes, holes injected from an electrode, to which a positive voltage is applied, may move to an light emitting layer via a hole transport layer, and electrons may move from an electrode, to which a negative voltage is applied, to the light emitting layer via an electron transport layer. The electrons and the holes recombine in the light emitting layer, thereby generating excitons. The excitons transition from an excited state to a ground state, thereby provoking the light emitting layer's fluorescent molecules to emit light and form an image.
The organic EL display operating as described above has an internal light efficiency and an external light efficiency. The internal efficiency depends on the organic luminescent material's photoelectric conversion efficiency. The external efficiency, which may be referred to as light coupling efficiency, depends on the refractive index of the display's layers. Organic EL displays may have lower external efficiency than other displays, such as cathode-ray tubes (CRTs), plasma display panels (PDPs), and field emission displays (FEDs). Accordingly, such organic EL displays need to be improved in terms of various characteristics of displays, such as brightness and life span.
In an organic EL display device shown in FIG. 1, as taught by Lu et al., in APL 78 (13), p. 1927, 2001, according to incident angle, light generated from an organic layer may experience total internal reflection at an interface between an indium tin oxide (ITO) layer and glass and at an interface between glass and air. In conventional organic EL display devices, the light coupling efficiency is typically about 23% or less, and the remainder of the light is not seen since it cannot escape the device.
In this regard, various methods for enhancing the light coupling efficiency of an organic EL display device have been proposed. A variety of methods associated with using diffractive gratings have been recently researched and reported.
Japanese Patent Publication No. hei 11-283751 discloses an organic EL display comprising a diffraction grating or a zone plate in a component in an organic EL element having one or more organic layers between a cathode and an anode.
Such an organic EL device may be complicated to fabricate since irregularities may need to be formed on a surface of a substrate or a fine electrode pattern layer, or a separate diffraction grating may need to be provided, thereby making it difficult to attain efficient productivity. Also, forming an organic layer on the irregularities makes the layer rougher, which may deteriorate the organic EL device's durability and reliability.
Korean Patent Publication No. 2003-0070985 discloses an organic EL display device including a light loss preventing layer, which has different refractive index regions between relatively large refractive index layers, among layers including a first electrode layer, an organic layer, and a second electrode layer. Referring to FIG. 2, the organic EL display device has a diffraction grating formed on a substrate, and light incident on the substrate is diffracted at an angle smaller than a critical angle of total internal reflection to be externally transmitted.
However, the conventional organic EL display's external light coupling efficiency largely depends upon a difference in the refractive index between diffraction grating layers. That is, the greater the refractive index difference, the higher the light coupling efficiency. Typically, materials having a low refraction index, i.e., substantially 1, tend to absorb moisture, which makes it difficult to utilize such materials in the fabrication process.